ST 165 not making full boost

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ST 165 not making full boost

Postby mpg » Wed Nov 30, 2016 3:05 am

Hello to All,

I'm in the process of getting my 1988 ST-165 ready for sale. It was recently serviced with top head work including a Cometic head gasket and full head rebuild to stock specs. Full compression leak down with no issues.
The car is all stock except for stock rebuild (Majestic Turbo, TX) and a new manifold (header) and downpipe - No TVIS or smog left on engine. HKS exhaust and adjustable timing gears and Magnecor plug wires.
Here's the problem:
When it ran before, no issue making full boost (the boost gauge needle would peg) and you knew it.
Now, no matter what gear, it won't go past 2/3 -3/4 on the gauge and you know it - Slow!
The repair shop said vacuum leaks, I sealed any and all - this place is 2 hrs away from me so I can;t just go there and wait, (long story)
So, they said block the wastegate lines and see if is makes full boost, which I did and it does (pegs the needle) and slams shut.
I asked what the engine timing was set to and was told zero on the exhaust, 1 degree negative on the intake.
My first thing I want to do is take the timing gear cover off and start changing the engine timing. Been there,done that, and I'll do it again. The car runs fine, just flat.
Any help would be greatly appreciated.
One last thing - I can't find anything about the plastic knob located on the firewall centered behind the intake that has the vacumn lines running from the turbo to the intake manifold - Is this a boost adjustment?
Thanks.
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mpg
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Re: ST 165 not making full boost

Postby underscore » Wed Nov 30, 2016 3:55 am

First of all hook up a proper boost gauge, it doesn't matter if it's a cheap mechanical one and you only use it temporarily (just run the hose out the hood and through the window) but get some real numbers, especially before disconnecting the lines to the wastegate. I'm guessing that the TVSV (which the ECU uses to up the boost in 3/4/5 gear) is either disconnected or got damaged during the rebuild and not doing its job. This would leave you running only the wastegate pressure which is ~6psi. If that's the case you can either fix/reconnect the TVSV or replace it with a boost controller.

Note that if the CEL is on at all the ECU will not trigger the TVSV so the car will run at low boost at all times.
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It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
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Re: ST 165 not making full boost

Postby audiodjmp77 » Wed Nov 30, 2016 4:24 pm

I thought if you disable tvsv, it allows full boost from the actuator. The plastic knob you are on about is, iirc to monitor map and bleeds off boost but cannot be sure

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1990 Toyota Celica

Re: ST 165 not making full boost

Postby underscore » Wed Nov 30, 2016 6:08 pm

Quite the opposite, the TVSV is a bleeder type boost controller that prevents the wastegate from seeing the full pressure, allowing higher boost before the wastegate opens.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
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Re: ST 165 not making full boost

Postby mpg » Wed Nov 30, 2016 6:47 pm

Yes, they could not get the CEL to turn off due to an idle speed issue so it was disabled and that sounds like why the car will not make full boost. If I disconnect the battery for a period of time, will it reset the CEL temporarily and give me the full boost back? I can then try to trace where it was disabled and connect it back up so I can read the error code(s) to see else is not working.
Thanks
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Re: ST 165 not making full boost

Postby underscore » Wed Nov 30, 2016 8:14 pm

It depends on how fast the error is detected by the ECU. Definitely reconnect your CEL, check the codes, and fix the problems. I can't remember the full list but the ECU does a few other things to reduce the boost level when the CEL is on so you definitely don't want any errors. My car would only make 4psi with a CEL.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
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